Add support for figcaption "title" option and keepAlt flag

[ItsEricWoodward]

Good day!

First off, thank you for the work that's been put into this excellent library.

This PR includes everything from #40, building off of Antonio-Laguna's excellent work, while also incorporating all of @arve0's suggestions and keeping backwards compatibility.

New Features

• Updates figcaption flag to take one of the strings alt or title, in addition to a boolean value (as now).

  • The title option provides the feature that Antonio-Laguna's code introduced, creating the figcaption from the image's title attribute rather than the alt attribute.
  • The alt option is the same as true, and provides the existing behavior (creating the figcaption from the image's alt attribute).

• Adds new keepAlt flag which takes a boolean value (defaults to false).

  • If set to true, and figcaption is set to true or alt, this keeps the alt attribute text in the img tag, rather than blanking it out (the existing / default behavior).

I also 8 new tests (and updated some existing ones) to cover the changes, and updated the README accordingly.

Fixes #20 & #39.

Please let me know if there's any changes I need to make. Thanks for your time!

[arve0]

Hi! 👋
Thanks for your work. I'm not using this library actively anymore, so I'm a bit hesitant to changes.

Seem like you've covered all cases, both previous tests and current tests pass with your changes 👍

I've released this as 0.12.0.

Thanks again